The Information Commissioner’s Office said it was probing the experiment and planned to ask Facebook questions after widespread outrage when it was revealed at the weekend. The regulator said it would also be in contact with the Irish data protection body, because Facebook has its European headquarters in Dublin.

A spokesperson for the ICO said it was too early to tell exactly what part of the law Facebook may have infringed. The regulator looks at how much personal data are used and whether users have given their consent.

Facebook won't be helped by the fact that the company didn't alter its terms of service to disclose to users that their posts would be used for research until four months after the experiment took place, according to Forbes:

Four months after this study happened, in May 2012, Facebook made changes to its data use policy, and that’s when it introduced this line about how it might use your information: “For internal operations, including troubleshooting, data analysis, testing, research and service improvement.” Facebook helpfully posted a “red-line” version of the new policy, contrasting it with the prior version from September 2011 — which did not mention anything about user information being used in “research.”